This invention relates to semiconductor diodes of the type known as varactors and more particularly to varactor diodes characterized by hyperabrupt PN junctions which exhibit very sharp C-V characteristics.
Hyperabrupt junction diodes are devices which are characterized by a rapid change in the depletion layer capacitance with reverse voltage. This is obtained, generally, by arranging the significant impurity on one side of the PN junction, usually the N-type to have a sharply decreasing concentration with distance away from the PN junction. Thus, there is enabled a rapid decrease in capacitance with reverse voltage because of the more rapid expansion of the depletion layer as compared to the response of linear graded and abrupt step junction diodes.
The disclosure of U.S. Pat. No. 3,638,300 to G. F. Foxhall and R. A. Moline describes a technique for fabricating a high performance hyperabrupt diode by ion implantation and diffusion of selected impurities into a relatively lightly doped epitaxial layer grown on a starting portion of monocrystalline silicon semiconductor material which is heavily doped to provide high conductivity.
Characteristics of particular importance with respect to hyperabrupt diodes used at microwave frequencies are the breakdown voltage and the rf series resistance. The first is, to some extent, a function of the total doping level particularly in the N-type portion near the PN junction. The implanted-diffused device, in which the significant dopants all are introduced from the epitaxial surface tends to have relatively high doping levels in this zone.
The series resistance is increased, disadvantageously, in a device having a thicker epitaxial layer and with more undepleted high resistivity silicon in the epitaxial portion between the PN junction and the epitaxial layer interface with the starting portion. The diode of the prior art which is produced by a process requiring relatively high temperatures for an appreciable time, tends to require a thicker epitaxial layer and as a result will have a thicker undepleted, high resistivity portion to allow for out-diffusion from the starting portion.
Accordingly, an object of this invention is a hyperabrupt diode in which the above-described problems are reduced.